The prior art astro navigation system incorporates an automatic astro tracker which computes the position of a selected celestial body relative to the vehicle in which the tracker is mounted, which searches out the body, which tracks the body automatically and accurately, and which determines the terrestrial position of the vehicle. The astro tracker includes a tracking telescope which during operation of the tracker is locked onto the celestial body. A vidicon or solid state stellar sensor is mounted in the tracking telescope, and an image of the celestial body is focused in the plane of the stellar sensor. By using a closed servo loop, the corrections from the tracker can be used. to correct the values of input latitude and longitude, so that latitude and longitude counters can be up-dated as long as the tracking telescope is locked onto the selected celestial body.
The tracking telescope in most astro trackers is gyro stabilized, such stabilization being achieved by mounting the telescope on a stable platform in an inertial measuring unit. The inertial measuring unit is a self-contained system which can automatically maintain angular reference directions in inertial space. The inertial measuring unit includes a platform supported, for example, on three gimbals. The tracking telescope is mounted on the platform, as are, for example, three single-axis gyros designated the X-gyro, the Y-gyro and the Z-gyro. Any drift of the platform from the attitude prescribed by the gyros causes one or more of the gyros to generate signals, each of which is applied in a corresponding servo loop to a torquer motor which, in turn, applies a correction torque to the corresponding gimbal to return the platform to its stabilized position. Accelerometers are also mounted on the platform to measure the acceleration of the vehicle along each of the three coordinate axes.
One of the problems encountered in inertial platform stabilized astro tracker navigational systems arises due to friction in the platform gimbal bearings and in the torquer motors. In such navigational systems, as mentioned above, a high accuracy tracking telescope containing a vidicon or solid state sensor is mounted on the platform of a stabilized gimbal set in an inertial measuring unit, the telescope providing line-of-sight tracking between the sensor and a selected celestial body. Movements of the platform in such an inertial measuring unit cause the gyros to develop error signals. As also described above, these error signals are used in associated servo systems to energize torquer motors which introduce torques to the gimbals of the inertial platform system in directions to nullify such movements so that a constant attitude may be maintained.
However, the torquer motors and the gimbal bearings have inherent friction so that the error signals developed by the gyros are not completely reduced to zero in the servo loop at the lower frequencies, and a slight residual angular motion of the tracking telescope persists which blurs the stellar image focused on the sensor therein. Specifically, friction of the gimbal bearings and torquer motors causes uncorrected angular motion of the inertial measuring unit to induce residual oscillations in the gimbal set which cannot be compensated by the servo loops associated with the gyros. Thus, the level of stabilization of the tracking telescope is limited by the aforesaid friction so that unstabilized motion of low frequency is coupled to the telescope which creates perturberations in the line-of-sight and resulting blurring of the stellar image focused on the sensor.
The present invention provides a vernier system which senses the gyro pick-off signals, and which uses these signals to stabilize the line-of-sight through the telescope to the sensor. This stabilization is achieved by introducing the gyro pick-off signals to piezo-electric crystals, and using the resulting changes in the size of the crystals to provide compensation in the optical system of the telescope. In brief, piezo-electric crystals are used as actuating devices for optical elements of the tracking telescope in order to achieve the desired purposes of the invention.
Attempts have been made in the past to control the secondary mirror of the folded optical system in the tracking telescope by solenoids, or the like, to achieve stabilization of the image focused on the sensor. However, such solenoids generate magnetic fields which adversely affect the operation of the sensor. The piezo-electric crystal actuators of the present invention, on the other hand, use remote, low value electric fields which have no adverse effect on the sensor. In addition, none of the prior art devices have the advantage of low power requirements, in any way comparable with the actuators of the present invention.
Briefly stated, the present invention provides a piezo-electric line-of-sight corrector which serves to move the secondary mirror in a folded-type optical system, or a planar mirror in a direct-type optical system, of a tracking telescope in an astro navigational system to compensate for spurious angular perturberational effects in the line-of-sight of the telescope due to incomplete stabilization of the inertial measuring unit in which the telescope is installed. The corrector provides fine calibration or adjustment under static conditions, as well as corrections for misalignment under dynamic conditions.
In a particular embodiment of the invention to be described, piezo-electric actuators are used to position the secondary mirror of a folded-type optical system in an inertial measuring unit stabilized telescope in one or two degrees of freedom. The piezo-electric crystals are formed as spider rods supporting the secondary mirror. Voltages representing, for example, the residual error signals from the gyros of the inertial measuring unit are applied to the piezo-electric crystal rods, and the resulting length differentials in the rods serve to change the angular direction of the line-of-sight in the telescope in a manner to compensate for the perturberances of the sensor.